Looking at the code it seems that this function is primarily targeted towards specifying alternate SSL socket implementation, rather than UNIX sockets. There are a number of socket factories in the PostgreSQL driver:
* org.postgresql.ssl.jdbc4.LibPQFactory
* org.postgresql.ssl.NonValidatingFactory
* org.postgresql.ssl.SingleCertValidatingFactory
and even an abstract base class:
* org.postgresql.ssl.DbKeyStoreSocketFactory
The Java Socket infrastructure is quite tied up to AF_INET and seems it is infeasible that UNIX Sockets will be supported via a SocketFactory.